This invention is in the field of auto-focusing techniques and relates to a method and an apparatus for determining focus error and for correcting focus errors.
There are a great variety of optical systems used for inspection/measurements of articles, reading/recording information in an information carrier, etc. These systems typically comprise an illumination means equipped with suitable light directing optics, and a detection means equipped with light collecting optics. A system of this kind always utilizes a focus error correction technique to ensure that the article under inspection (or an addressed layer of the information carrier) is located in a focal plane defined by the light directing optics.
U.S. Pat. No. 6,124,924 of Feldman et al. (which is incorporated by reference) describes a method and apparatus for maintaining a desired position of an article, the apparatus includes: (a) an illumination apparatus generating a light beam for illuminating an elongated region of the article and producing light returned from the illuminated region; (b) a focusing optics for focusing the beam onto a focal plane and collecting at least a portion of the returned light; (c) a focus detection apparatus comprising an imaging optics and a detector, having a sensing surface responsive to light impinging thereon for generating data representative thereof, the imaging optics receiving at least a part of the collected returned light and forming first and second images on said sensing surface, wherein the first and second images are formed by first and second spatially separated substantially identical light components of the collected returned light, respectively, propagating symmetrically relative to an optical axis of said focusing optics, a position of the first image relative to the second image being indicative of the position of the article relative to the focal plane.
The apparatus of Feldman et al. offers a compromise between the focus error dynamic range and focus error sensitivity. The focus error dynamic range is limited by the size (actually height) of the sensing surface and a ratio between the focus error and the relative distance between the first and second images that result from the focus error. The focus error sensitivity is inversely proportional to the focus error dynamic range. An increase in the focus error dynamic range must be compensated by a corresponding decrease in the focus error sensitivity. The dynamic range may also be increased by increasing the sensing surface, but this may increase the size and complexity of the apparatus.
There is a need to provide a focus error detection apparatus that is characterized by large dynamic range and high sensitivity.
The invention provides a focus error detection apparatus operable to receive light reflected from an inspected object and to determine a focus error on a surface of the inspected object, the focus error detection apparatus including optics and at least one detection apparatus, whereas the optics and the at least one detection apparatus define at least a first focus error detection path and a second focus error detection path, the first focus error detection path detects focus errors with a greater sensitivity than the second focus error detection path, while the second focus error detection path detects focus errors over a greater focus error range than the first focus error detection path.
The invention provides a focus error detection method that includes the steps of: (i) receiving light that was reflected from an inspected object located at a inspected object plane; (ii) passing a first light component of the reflected light through a first focus error detection path and passing a second light component of the reflected light through a second focus error detection path; and (iii) determining a focus error in response to detection signals generated by the first and second focus error detection paths. Wherein the first focus error detection path detects focus errors with a greater sensitivity than the second focus error detection path, while the second focus error detection path detects focus errors over a greater focus error range than the first focus error detection path.
The focus error is determined in response to a location of either a first image or a second image that are formed by the first or second light component passage through the first and second focus error correction paths. Conveniently, a reference image reflecting the position of the first and second images in case of in-focus is also imaged upon the detection apparatus, and the distance between the reference image and the image reflects the focus error.
Conveniently, the first focus error detection path has a lower xcex94Z/xcex94xcex8 ratio, whereas xcex94Z reflects the distance between an image and a reference image and xcex94xcex8 reflects the orientation of the first light component to an optical axis of the focus error detection apparatus. When in-focus both xcex94Z and xcex94xcex8 are substantially zero.